A communication partner device of the kind specified in the second paragraph above having a signal processing circuit of the kind specified in the first paragraph above has been put on the market in a number of variant designs by the applicant and the device and circuit in question are therefore known. In the known communication partner device, the transceiver circuit is formed by an integrated circuit or IC, which IC has been put on the market by the applicant under the type number MF RC500. The IC used as a transceiver circuit has two transmitted-signal outputs and one received-signal input. Connected to each of the two transmitted-signal outputs is a transmitted-signal input of the signal processing circuit. Connected to the received-signal input of the IC is a received-signal output of the signal processing circuit. The signal processing circuit includes, on its transmitted-signal path, a filter stage that forms what is termed an EMC filter and is intended to ensure the electromagnetic compatibility laid down by the authorities. Connected to the filter stage is a matching stage that comprises a combination of two series capacitors and two parallel capacitors and by means of which the impedance of a transmission coil connected to the signal processing path can be set to a given desired value. Connected to the matching stage is a damping stage that is formed by means of two resistors, the above-mentioned transmission coil being connected to the damping stage. The quality factor of the transmission coil can be set to a required desired value by means of the damping stage.
In the known design, the filter stage is sized in such a way that the value of the resonant frequency of said filter stage when tolerances are ignored, i.e. the nominal value of the resonant frequency, corresponds to the operating frequency, which operating frequency is for example 13.56 MHz. Also, in the known design, the branch point is provided between the matching stage and the damping stage, meaning that the received signal emitted by the transmission coil is picked off immediately downstream of a resistor in the damping stage.
A result of the design described for the known signal processing circuit is that the filter stage contains an inductor of relatively high inductance, as a result of which a relatively large inductor has to be provided to enable a sufficiently high inductance of approximately 1 μH to be obtained. To obtain a capacitance suited to the inductance of 1 μH, and hence to obtain the desired resonant frequency value, two capacitors connected in parallel have to be provided, which constitutes a relatively high cost. Also, there is the problem with the known design that when steep signal edges occur, which happens when signals in pulse form are being transmitted, a critical overshoot phenomenon occurs, which is a disadvantage. Another thing that has to be said about the known design is that the frequency characteristic that exists in the receiving mode is relatively narrow-band, which proves to be particularly disadvantageous in many applications.